The present invention relates to a fuel assembly and a nuclear reactor, and more particularly to a fuel assembly suitable for use in a high conversion area of a light water moderation type nuclear reactor and a boiling water reactor having a high conversion area.
Methods of utilizing nuclear fuel in a light water moderation type nuclear reactor (hereinafter referred to as a light water reactor) are roughly classified into a "once-through" method and a reprocessing or recycling method. With the "once-through" method, the light water reactor uses the enriched uranium and in this method, none of the fuel materials contained in the used fuel rods which are taken out of the light water reactor is reused or recycled in the light water reactor. The "once-through" method or system is advantageous in terms of the fuel running cost in the case where the cost of reprocessing fuel is higher than that of enriching uranium.
One method to effectively use the fuel materials by the "once-through" method is to greatly increase the take-out burnup from the fuel assembly, that is, to realize a high efficiency of the burnup. It is required to raise the enrichment of the uranium-235 to achieve the high degree of the burnup, but the raised enrichment of the uranium would suffer from the following problems: In the center of the reactor core of the light water reactor there are the fuel assemblies with large difference in the neutron infinite multiplication factor because of a high enrichment of the new fuel assemblies and the large take-out burnup, thus developing a difference in the output power share proportions of the individual fuel assemblies accompanied by the larger output power mismatch and the increased output power peaking. Furthermore, as the enrichment increases, the surplus reactivity which has to be controlled in the initial stage of the burning increases.
Japanese Patent Unexamined Publication No. 129594/1986 (which has a U.S. equivalent, U.S. patent application Ser. No. 800,266 filed on Nov. 21, 1985 and a European equivalent, EPC laid-open application No. 184134 published on June 11, 1986) shows boiling water reactors and pressurized water reactors which are light water reactors having reactor cores to eliminate the above-described problems and to realize a high degree of burnup through highly enriched uranium. These reactor cores are used to improve the conversion of uranium-238 that is the fuel fertile material into nuclear fissile material (plutonium-239, further to effectively burn the nuclear fissile plutonium produced within the nuclear cores and the enriched uranium-235 and to effectively utilize the nuclear fuel material according to the oncethrough method. More specifically, as shown in FIG. 1, a reactor core 1 is divided in the radial direction into two area by a tubular partition member 2, and these areas are different from each other in ratio of the number of uranium atoms to that of hydrogen atoms (the ratio will hereinafter be referred to as .gamma.H/U). Such a reactor core 1 is composed of fuel assemblies A each having a small ratio .gamma.H/U (=1.0) as shown in FIG. 2 and fuel assemblies B each having a large ratio .gamma.H/O (=5.0), these assemblies A and B being charged in the burnup region. Each of the fuel assemblies A and B has a number of fuel rods 3 arranged in a regular triangular lattice. The fuel assembly B is provided with burnable poison rods 4. In contrast, the fuel assembly A is not provided with any burnable poison rod. The fuel assemblies A are charged or loaded in the high conversion area inside the tubular partition member 2 in a first half of the reactor core entire life, and are reassembled into the fuel assemblies B. Thereafter, the fuel assemblies B are charged or loaded in the burnup area outside the tubular partition member 2 in the final half of the reactor core entire life. In other words, the fuel assemblies are loaded during the initial half stage of the reactor core entire life in the area where .gamma.H/U is small and the neutron spectrum is hard (high conversion area) to convert the fuel fertile material into the nuclear fissile material, and are loaded in the final half stage of the reactor core entire life in the area where .gamma.H/U is large and the neutron spectrum is soft (burner area) to effectively burn the nuclear fissile material. The dependency of the neutron infinite multiplication upon the burnup during these processes is shown in FIG. 4. In the high conversion area in which the highly condensated uranium fuel, that is, the new fuel assembly is loaded, the neutron infinite multiplication is low, whereas, in the burner area in which the half-burnt fuel assembly is loaded, the neutron infinite multiplication factor is high. Therefore, it is possible to reduce the output mismatch and to suppress the surplus reactivity of the new fuel assemblies.